Integrated process for the preparation of aromatic isocyanates and procedures for effecting the relative intermediate phases

ABSTRACT

Integrated process for the preparation of aromatic isocyanates comprising: the reaction between an aromatic amine and an organiccarbonate in the presence of a catalyst selected from organic and inorganic salts of a metal selected from Zn, Sn, Pb, Cu; the removal of the catalyst; the passivation of the quantity of residual metal in the urethane formed in step a); the removal of the organic solvent and its optional recycling to step a) of the reaction; the evaporation of the aromatic urethane with partial pyrolysis; the complete pyrolysis of the urethane in gas phase; the recovery of the isocyanate.

[0001] The present invention relates to a process for the preparation ofaromatic isocyanates consisting in the conversion of an aromatic amineinto the corresponding urethane by reaction with an organic carbonate,in the presence of a suitable catalyst, and in the subsequent thermaldecomposition of the urethane groups into isocyanate, after anintermediate series of operations such as the removal of the catalyst,the passivation of the metal residues in the urethane, and theevaporation of the latter; the invention also relates to the specificprocedures for effecting these intermediate phases.

[0002] The preparation of isocyanates starting from the reaction betweenamines and organic carbonates is known.

[0003] For example, U.S. Pat. No. 5,315,034 describes a multistepprocess for the preparation of alkyl mono and diisocyanates consistingin reacting acting the corresponding aliphatic amine or diamine withdimethylcarbonate and, substantially, in vaporizing and partiallyconverting the urethane thus formed in an evaporator, subsequentlyterminating the cracking in a second reactor, and finally subjecting thecracking product to fractionated distillation at reduced pressure, withrecycling of the non-converted part to the partial vaporization step; inthe first phase a base catalyst is used, consisting of alcoholates ofalkaline or earth-alkaline metals: the process allows alkyl mono anddiisocyanates to be obtained with good yields, but is strictly limitedto these and it does not seem that the disclosure can be easily extendedto aromatic isocyanates.

[0004] The process described in International patent application WO98/56758 proposes a widening in the spectrum of isocyanates preparedwithout the use of toxic agents and essentially consists in the reactionbetween an amine and an organic carbonate in the presence of a catalystand organic solvent, the removal of the catalyst, the thermaldecomposition of the carbamate formed and final distillation of thesolvent/alcohol mixture.

[0005] The process however does not seem to be of industrial interest asit has a low productivity owing to the fact that the thermaldecomposition is carried out in a solvent under dilute conditions.

[0006] The Applicant has now found that by using a well defined group ofmetal catalysts and operating conditions in the initial reaction betweenaromatic amine and organic carbonate, by specifically treating the metalresidues present in the urethane after the normal removal of thecatalyst and causing the pyrolysis of the urethane in gas phase, afterits evaporation, it is possible to obtain high productivities ofaromatic isocyanates with a high degree of selectivity.

[0007] A first object of the present invention in fact relates to anintegrated process for the preparation of aromatic isocyanatescomprising:

[0008] a) the reaction between an aromatic amine and an organiccarbonate in the presence of a catalyst selected from organic andinorganic salts of a metal selected from Zn, Sn, Pb, Cu;

[0009] b) the removal of the catalyst;

[0010] c) the passivation of the residual quantities of metal in theurethane formed in step a);

[0011] d) the removal of the solvent and its optional recycling to stepa) of the reaction;

[0012] e) the evaporation of the aromatic urethane with partialpyrolysis;

[0013] f) the complete pyrolysis of the urethane in gas phase;

[0014] g) the recovery of the isocyanate.

[0015] The process for the preparation of aromatic isocyanates accordingto the present invention can be described in detail as follows.

[0016] In the first step, the process for the synthesis of aromaticurethanes comprises:

[0017] reacting an organic carbonate in stoichiometric quantities, orhigher than the stoichiometric value, with an amine having formula (I):

R—(NH₂)_(n)  (I)

[0018] wherein n is an integer ranging from 1 to 2, R represents an arylradical, such as monovalent, bivalent radicals of benzene, toluene,naphthalene, diphenyl, methylene-diphenyl.

[0019] The aryl radical can contain, as substituents, one or more alkylradicals having from 1 to 4 carbon atoms.

[0020] The aryl radical can contain, as substituents, atoms or radicalswhich are non-reactive with the isocyanate function, such as halogenatoms, alkoxy, nitro, cyano, acyl, acyloxy, isocyanate groups.

[0021] Non-limiting examples of aromatic amines having formula (I) are:2,4-diaminotoluene, 2,6-diaminotoluene or mixtures of the two isomers,aniline, toluidine, 3,5-di-chloroaniline, 4,4′-methylenedianiline,2,4′-methylenedi-aniline, 2,2′-methylenedianiline or mixtures ofisomers.

[0022] The reaction is carried out in the presence of a catalystselected from organic or inorganic salts, of a metal selected from Zn,Sn, Pb, Cu: various salts are used. Among these however anhydrous ordihydrate zinc carboxylates, base carbonates of copper, base carbonatesof zinc, mixed carbonates of zinc and copper, zinc carbamates, arepreferred.

[0023] The alcohol which is formed during the reaction is continuouslyremoved by distillation, maintaining inside the reaction mixture aquantity thereof ranging from 10 to 40% with respect to the totalco-product.

[0024] Organic carbonates which can be used in the process are alkylesters of carbonic acid. The ester group contains an alkyl group with upto 6, preferably up to 4, carbon atoms. Examples of particularlysuitable organic carbonates are dimethyl carbonate, diethyl carbonate,dipropyl carbonate. The organic carbonates can be prepared using theknown methods. The quantity of carbonate used varies from thestoichiometric value with respect to the amine groups contained in themolecules having formula (I), to an excess quantity, as the carbonatecan be used as solvent.

[0025] Any solvent can be used provided it is inert with the reagentsunder the operating conditions. Mixtures of suitable solvents can alsobe used.

[0026] The solvents can be selected from alkylated and non-alkylatedaromatic hydrocarbons, such as for example, benzene, toluene, xylene;aromatic hydrocarbons containing functional groups inert with thereagents, such as for example, anisole, benzonitrile, chlorobenzene,dichlorobenzene; alkanes and alkanes containing functional groups inertwith the reagents, such as for example, cyclohexane, n-heptane,n-hexane, dichloromethane, diethylether, acetonitrile, dioxane.

[0027] The quantity of catalyst can vary from 20 to 0.5% in moles,preferably from 10 to 1.0 in moles per mole of amine (I). The reactiontemperature can vary from 100 to 200° C., preferably from 140-180° C.,and can be kept constant or increased within the above range, during thereaction.

[0028] The reaction is carried out at an operating pressure orautogenous pressure of the system, or in any case ranging from 2 to 15absolute atm., preferably from 3 to 7 absolute atm.

[0029] The reaction time is in relation to the temperature and pressure:however, reaction times ranging from 1 to 5 hours have proved adequate.

[0030] The reaction proceeds until the complete, or substantiallycomplete, conversion of the amine groups to form a mixture of aromaticurethane and alcohol, this being removed alone or in a mixture with theexcess organic carbonate, and the mixture is separated according to theconventional techniques, with the possible organic carbonate which isrecycled to be fed again to the formation reaction of urethane.

[0031] The catalyst is subsequently removed, using for the purpose anymethod known to experts in the field. It has also proved to beparticularly advantageous to remove the catalyst, after the addition ofwater, according to a procedure which, as it represents an importantaspect of the process for the preparation of aromatic isocyanatesaccording to the present invention, forms a second object thereof, thuscharacterizing a very particular process for the preparation of aromaticurethanes.

[0032] The addition of water allows the metal residues, which accompanythe urethane after filtration, to be kept within quantities of less than20 ppm, with obvious advantages considering that the traditional methodsfor the removal of catalysts do not reduce the metal residues in theurethane to below 500÷1000 ppm.

[0033] The water, in quantities ranging from 0.5:1 to 10:1 moles ofwater per mole of catalyst initially charged, preferably from 1:1 to 4:1moles, is added directly to the reaction mixture at temperatures rangingfrom 100 to 200° C., preferably from 110 to 160° C., at pressuresranging from 2 to 15 absolute atm., preferably from 3 to 7 absolute atm.

[0034] A second important object of the present invention thereforerelates to a process for the synthesis of aromatic urethanes whichcomprises reacting an organic carbonate and an aromatic amine, accordingto the terms and conditions described above, in the presence of one ofthe above-mentioned catalysts or other metal derivatives, and which ischaracterized by the addition of water to remove the catalyst before therecovery of the urethane formed.

[0035] The conditions of the addition of water, in the above process forthe preparation of aromatic urethanes, are those which have just beendescribed in relation to this procedure when selected as step b) of theprocess for the preparation of aromatic isocyanates according to thepresent invention.

[0036] With reference to this process, it is important, for effectingthe subsequent steps and obtaining results which are industriallyinteresting, for the urethane obtained after the removal of the catalystto be subjected to a particular treatment to enable the final thermaltreatment to be carried out, reducing to the minimum the effect ofside-reactions such as decarboxylation of the urethane with theformation of an amine; the reaction between the amine and isocyanateproduced with the formation of ureas, the reaction between theisocyanate produced and the starting urethane with the formation ofallophanates and polymeric products.

[0037] The undesired reactions which result in the formation of theseby-products are so strongly favoured by the presence in the startingurethane of metal catalytic residues, deriving from the productionreaction of the urethane itself, as to cause fouling of the equipmentnecessitating interruption of the processing.

[0038] According to the present invention, it has been unexpectedlyfound that it is possible to evaporate with partial pyrolysis and topyrolyze in gas phase the aromatic urethanes containing catalyticresidues, with high yields and high selectivities to isocyanates afterstabilizing treatment with phosphoric acid or oxalic acid: this processhas a general significance, and forms a third object of the presentinvention, in that it is an important step in the process for thepreparation of aromatic isocyanates.

[0039] More specifically, the procedure for the production of aromaticisocyanates is characterized in that aromatic urethanes containingcatalytic residues are dissolved in a low-boiling solvent, present in aweight ratio with respect to the urethane ranging from 1:1 to 10:1,preferably from 2:1 to 6:1 and are treated with phosphoric acid in amolar ratio of the latter with respect to the metal, present ascatalytic residue, ranging from 1:1 to 10:1 and preferably from 1:1 to3:1 at a temperature ranging from 100° C., preferably from 120° C. to140° C., at a pressure ranging from 2 to 15 absolute atm., preferablyfrom 3 to 7 absolute atm., for a period of time ranging from 0.5 to 4hours, preferably from 1 to 2 hours.

[0040] The treatment can be carried out directly on the solution ofurethane coming from the synthesis, after separation of the catalyst(step b).

[0041] At the end of the treatment, the solvent is eliminated bydistillation at reduced pressure. Examples of solvents are:dimethylcarbonate, diethylcarbonate, tetrahydrofuran, dioxane,acetonitrile, methanol, ethanol; the solvent in which the synthesisreaction of urethane is effected, is preferred.

[0042] The treated urethane is then subjected to evaporation andpyrolysis in gas phase.

[0043] With reference to the process in question, at the end of thepassivation procedure of the catalyst, the solvent is removed bydistillation in an apparatus consisting of one or more evaporators,which operate with short contact times, at a temperature ranging from100° C. to 200° C., preferably from 150° C. to 180° C., at a pressureranging from 3 to 0.2 absolute atm., preferably from 2 to 0.5 absoluteatm.

[0044] The solvent recovered is optionally recycled to the synthesisreaction of step a) and the aromatic urethane, in accordance with theprocess for the preparation of aromatic isocyanates according to thepresent invention, is subjected to evaporation with partial pyrolysisand to subsequent pyrolysis in gas phase. The above-mentioned procedure,which, when applied to the process according to the present invention,represents the concluding phase, can, in turn, form a furthersignificant object of the present invention as it represents, in generalterms, a procedure for the preparation of aromatic isocyanates by meansof the pyrolysis of the corresponding urethanes, however obtained, saidprocedure consisting in subjecting the urethanes in molten state or insolution, to thermal treatment under such conditions of temperature andpressure as to allow the evaporation and partial conversion intoisocyanates and, subsequently subjecting the vapors to a second thermaltreatment in a pyrolysis reactor, which operates at a higher temperatureor at a value however that is sufficient to complete the conversion ofthe urethane.

[0045] It is evident that the combined effect of the evaporation of theurethane and its subsequent pyrolysis would be much more significant ifthe urethane had been previously subjected to the above passivationtreatment of the catalytic residues.

[0046] The vapor leaving the pyrolysis reactor, which contains alcoholand isocyanate, is subjected to fractionated condensation to separateits constituents.

[0047] Compared with the known methods, the process described aboveallows isocyanates to be obtained with a high volume/time yield and ahigh selectivity, using a method which can be easily industrialized,allowing operation in continuous.

[0048] More specifically, the procedure for the production ofisocyanates having formula III is characterized in that urethanes havingformula II are thermolithically cracked according to the followingequation:

R(NHCOOR′)_(n)→R(NCO)_(n)+nR′OH II  III

[0049] wherein n and R have the same values and meanings as formula I;R′ represents an aliphatic organic radical, containing from 1 to 6carbon atoms, preferably containing from 1 to 4 carbon atoms.

[0050] Typical examples of urethanes which are pyrolyzed in the processof the present invention are: 2,4-toluene dimethylurethane, 2,6-toluenedimethylurethane, mixtures of 2,4-toluene dimethylurethane and2,6-toluene dimethylurethane, N-phenylmethylurethane,N-phenylethylurethane, 4-methylphenylethylurethane, 3,5-dichlorophenylethylurethane, 4,4′-methylene bis(phenylmethylurethane),2,4′-methylene bis(phenylmethylurethane), 2,2′-methylenebis(phenylmethyl-urethane) or mixture of isomers.

[0051] An object of the present invention therefore also relates to aprocess for the preparation of aromatic isocyanates by the pyrolysis ofurethanes in gas phase, characterized in that urethanes in the moltenstate, or in solution with an inert high-boiling solvent, are fed to afirst pyrolysis reactor, which causes the partial or total evaporationof the urethane fed and a partial pyrolysis thereof, operating at atemperature within the range of 230° C.-380° C., preferably within therange of 270° C.-320° C., at a pressure within the range of 1 to 300mmHg, preferably within the range of 20-150 mmHg.

[0052] The liquid urethane is fed to the first pyrolysis reactor with aLSHV space velocity ranging from 0.2 to 4 hours⁻¹, preferably from 0.5to 2 hours⁻¹.

[0053] The mixture of vapors leaving the first reactor, which containsin addition to the starting urethane, also pyrolysis products, is fed tothe second pyrolysis reactor which operates at a temperature rangingfrom 300° C. to 600° C., preferably from 350° C. to 550° C. and is inequi-pressure with the first pyrolysis reactor.

[0054] The feeding of the vapors to the second cracking reactor takesplace with a GHSV space velocity, under normal conditions, ranging from20 to 500 hours⁻¹, preferably from 40 to 200 hours⁻¹.

[0055] The mixture of vapors leaving the second cracking reactor issubjected to fractionated condensation, with a first condensation at atemperature within the range of 10° C.-150° C., preferably within therange of 20° C.-100° C., allowing a fraction containing the desiredisocyanate to be obtained, and a second condensation at a temperaturewithin the range of −80° C. to +50° C., preferably within the range of−30° C. to +10° C. from which a fraction mainly containing alcohol isobtained.

[0056] The high-boiling inert solvent, which optionally dilutes theliquid urethane fed, can be present in a weight ratio with respect tothe urethane ranging from 3/1 to 0.01/1, preferably from 0.3/1 to0.05/1.

[0057] The solvent must be inert under the reaction conditions, itshould have a boiling point higher than that of the urethane andpreferably has good solvent properties for the urethane.

[0058] Examples of these solvents are: substituted or non-substitutedaromatic hydrocarbons such as polyphenyls, triphenyl, tetraphenyl,dodecylbenzene, dibenzyltoluene, polyphenylether, methylnaphthalene,benzylnaphthalene, dichloronaphthalene, esters of organic acids such asdibutylphthalate, dioctylphthalate, sulfones such as diphenylsulfone,phenyltolylsulfone, naphthylphenylsulfone.

[0059] The first pyrolysis reactor can be a fine film evaporator inwhich, with the supply of an appropriate quantity of heat, the productfed can be completely vaporized and already partially converted toisocyanate; or, preferably, a fraction mainly containing urethane can bedischarged from the bottom to obtain a solvent effect on the smallquantity of polymeric products present. The ratio between vaporizedproduct and product collected at the bottom ranges from 70:30 to 99:1,preferably from 80:20 to 95:5.

[0060] In an embodiment of the invention, the product collected at thebottom, after separation from the pitches, is re-fed to the firstpyrolysis reactor. The separation of the pitches is carried out byextraction of the urethane in a solvent in which the pitches areinsoluble, or by evaporation of the urethane in a subsequent apparatusin which the pitches are collected at the bottom.

[0061] Examples of solvents which can be used for the extraction of theurethane are: methanol, ethanol, propanol, butanol, acetonitrile,tetrahydrofuran, dioxane, chloroform, methylene chloride,methylpyrrolidone. The preferred solvent is the alcohol corresponding tothe R′ alkyl group in formula (II) of urethane.

[0062] If the urethane, fed to the film evaporator, is diluted in asolvent with a boiling point higher than that of the urethane, themolten liquid, discharged from the bottom, mainly contains the solventwhich, after separation from the polymeric products, can be optionallyrecovered.

[0063] The second pyrolysis reactor is generally a quartz or inox steel,cylindrical, tubular reactor. This reactor can be used empty or filledwith heat-resistant material, such as steel chips or rings or otherfillings known in the art, which however have the effect of improvingthe heat transfer.

[0064] The aromatic isocyanate obtained is recovered from the condensedfraction in which it is contained as main product in a concentrationnormally ranging from 80% to 99% by weight, by means of continuous orsemi-continuous distillation in an apparatus consisting of anevaporator, which operates with short contact times, and a column.

[0065] This apparatus operates at a temperature within the range of 60°C.-200° C., preferably within the range of 90° C.-150° C. and at apressure within the range of 1 to 200 mmHg, preferably within the rangeof 3 to 40 mmHg.

[0066] The condensed vapors, obtained at the head of the column in aratio with the product fed normally ranging from 0.5/1 to 0.95/1,contain isocyanate with a purity of over 99.5% parts by weight.

[0067] In an embodiment of the invention, the liquid remaining at thebottom of the column, which contains isocyanate, products containingurethane functions and by-products of the ureic type, is recycled to thefirst pyrolysis reactor.

[0068] This fraction is fed directly to the first pyrolysis reactor, oris first treated with the alcohol corresponding to the R′ alkyl group informula (II) of the urethane to transform the isocyanate functions intourethane functions and is then fed to the above reactor.

[0069] This treatment is carried out in a stirred reactor at atemperature within the range of 25° C. to 90° C. for a time ranging from0.5 to 3 hours, using the alcohol in a weight ratio with the liquid tobe treated ranging from 2:1 to 10:1.

[0070] The integrated process for the preparation of aromaticisocyanates according to the main object of the present invention, aswell as the procedures for effecting the intermediate phases thereof,which also apply to contexts outside that defined herein, and, in turn,also object of the present invention, can be more clearly understoodfrom the following examples which are provided for illustrative purposesand do not limit the scope of the invention.

EXAMPLE 1 Synthesis of Toluenediurethane from the CorrespondingToluenediamine

[0071] With reference to FIG. 1, 165 g (1.352 moles) of toluenediamine80/20 (TDA 80/20, mixture of 2,4- and 2,6-isomers in a proportion of80/20), 1600 g of dimethylcarbonate (DMC, weight ratio DMC/TDA equal to9.7) and 8.9 g of zinc acetate dihydrate (0.040 moles, 5.4% by weightwith respect to the TDA 80/20, molar ratio catalyst/TDA 80/20 equal to0.03), are charged into a cylindrical steel autoclave (A) with a usefulvolume of 3 liters. The autoclave is then pressurized with nitrogen or2.5 absolute atmospheres, heated so as to maintain an internaltemperature of 160° C. for 1.5 hours and stirred at about 300 rpm.

[0072] The condenser B is brought to a temperature of 120° C., whereasthe condenser C is cooled to a temperature of 5° C.

[0073] After 1.5 hours the connection between the autoclave and thecondenser B is intercepted, the internal temperature is brought to 175°C. for the following 2 hours (finishing phase).

[0074] The maximum pressure registered during the test proved to beequal to 9 absolute atm.

[0075] The reaction is left to cool to 120° C. and a quantity of waterequal to 1.5 g is added. The mixture is maintained under stirring at120° C. for about 30 minutes, after which filtration is effected on anF1 sintered steel filter, having an average pore diameter equal to 2microns (pressure about 5 absolute atm.).

[0076] In this way, after evaporation at reduced pressure of the solventDMC, a raw product is obtained having a weight of 317 g, consisting of302 g of toluenediurethane 80/20 (mixture in a proportion 80/20 of therespective carbamates of TDA 80/20) and a mixture of by-products havinga weight of 15 g.

[0077] At the end of the reaction, a quantity of distillate equal to 350g was collected, of which 86.5 g of methanol, 6 g of methyl acetate and257.5 g of dimethylcarbonate.

[0078] From these results, the following yield, conversion andselectivity values can be calculated:

[0079] conversion with respect to the starting TDA 80/20>99%

[0080] selectivity to toluenediurethane 80/20 equal to 94%

[0081] yield equal to 94%.

[0082] The content of metal zinc residue in the urethane proved to be 15ppm.

EXAMPLE 2 Comparative

[0083] The reaction is carried out under the same operating conditionsas example 1, using the same quantity of reagents.

[0084] At the end of the reaction, the autoclave is cooled to about 120°C., filtering on a filter thermostat-regulated at the same temperatureas the autoclave.

[0085] The raw product collected is subsequently distilled at reducedpressure to completely remove the solvent. In this way, a solid residueis obtained of about 270 g. From the HPLC analysis effected, it ispossible to calculate the following yield, conversion and selectivityvalues:

[0086] conversion with respect to the starting TDA 80/20≧99%

[0087] selectivity to toluenediurethane 80/20 equal to 94%

[0088] yield equal to 94%.

[0089] The content of metal zinc residue in the urethane proved to be1000 ppm.

EXAMPLE 3 Passivation Treatment of the Urethane Catalytic Residues

[0090] 300 g of toluene dimethylurethane (hereafter TDU) containing 15ppm of zinc, having a titer of 94% by weight and containing the two 2,4toluene dimethylurethane and 2,6 toluene dimethylurethane isomers in aweight ratio 80/20, are charged into a cylindrical steel autoclave,having a useful volume of 3 liters.

[0091] 1650 ml of dimethylcarbonate and 12 mg of H₃PO₄ at 85% are alsocharged into the autoclave and the mixture is maintained under stirringfor 2 hours at a temperature of 130° C.

[0092] At the end of the treatment, the autoclave is cooled, emptied andthe solvent is removed by distillation at reduced pressure.

EXAMPLE 4 Pyrolysis of Urethane in Gas Phase

[0093] The pyrolysis step is carried out in the apparatus, illustratedin FIG. 2, consisting of:

[0094] 1. A melter (V1) of the urethane.

[0095] 2. A dosage pump of the urethane (P1).

[0096] 3. A fine film evaporator (EV1) in which the urethane isevaporated, with an exchange surface of 2.2 dm².

[0097] 4. A tubular cracking reactor (R1) made of Aisi 316L stainlesssteel having a length of 1000 mm, an internal diameter of 24.8 mm,filled with chips of the same material.

[0098] 5. A condenser (EC1) for the condensation of the isocyanate.

[0099] 6. A condenser (EC2) for the condensation of the methanol.

[0100] 300 g of TDU (titer 94%) treated with H₃PO₄ as described inexample 3, are charged into and melted in the melter V1, at atemperature of 175° C. and at atmospheric pressure.

[0101] The condenser EC1 is cooled with cooling liquid at T=40° C., EC2is cooled with cooling liquid at T=−20° C., EV1 is heated withdiathermic oil at T=306° C. and R1 is heated electrically at a walltemperature of 456° C.

[0102] EV1, R1, EC1, EC2, V2, V3, V4 are brought to residual pressurevalues of 70 mmHg, measured at the outlet of EV1.

[0103] Operating under these conditions, molten TDU is fed by means ofP1, with a flow-rate of 297 g/hour and 279 g/hour of mixture areevaporated in EV1.

[0104] A liquid stream, equal to 18 g/hour, having the followingcomposition, is collected in V2 from the bottom of EV1:

[0105] 48.5% by weight of TDU

[0106] 14% by weight of toluene monourethane monoisocyanate (TMI)

[0107] 37.5% by weight of heavy by-products (allophanates, ureas).

[0108] The gas leaving R1 (279 g/hour) is partially condensed in EC1 andthe liquid obtained, equal to 206 g/hour, collected in V3, has thefollowing composition:

[0109] 89.7% by weight of toluene diisocyanate (TDI)

[0110] 4% by weight of TMI

[0111] 6.2% by weight of ureas

[0112] 0.1% by weight of TDU

[0113] The non-condensed gas in EC1, equal to 73 g/hour, which mainlycontains methanol, is condensed in EC2 and collected in V4.

[0114] From the above data, a conversion of TDU equal to 96.8%, aselectivity to TDI of 93.5% and a selectivity to TMI of 4.6%, arecalculated, for the pyrolysis.

EXAMPLE 5 Recovery of the Isocyanate

[0115] In the final step the TDI is recovered from the mixture collectedin V3 of example 4, by means of distillation in the apparatusillustrated in FIG. 2, consisting of:

[0116] 1. A fine film evaporator (EV2), having an exchange surface of 2dm², in which the mixture to be rectified is vaporized.

[0117] 2. A pump (P2) for the feeding of this mixture to EV2.

[0118] 3. A distillation column (C1), having an internal diameter of 25mm, a length of 500 mm, filled with Wilson coils.

[0119] 4. A condenser (EC3) of the isocyanate.

[0120] The condenser EC3 is brought to T=20° C., EV2 is heated to T=125°C. and the whole distillation equipment consisting of EV2, C1, EC3, V5,V6, is brought to a residual pressure value of 5 mmHg.

[0121] Operating under these conditions, the mixture contained in V3 isfed to EV2 with a flow-rate of 70 g/hour. The vapor leaving C1 (49g/hour) is completely condensed in EC3 and the liquid obtained,collected in V6, consists of TBI with a purity of over 99.53 by weight.

[0122] A liquid stream, equal to 21 g/h, having the followingcomposition, is collected in V5 from the bottom of EV2:

[0123] 66.8% by weight of TDI

[0124] 12.2% by weight of TMI

[0125] 20.7% by weight of ureas

[0126] 0.3% by weight of TDU.

[0127] From the above data, a recovery yield of TDI equal to 77.5% perpassage and a mass balance of the isocyanate equal to 100%, arecalculated.

EXAMPLE 6 Comparative

[0128] With reference to the equipment illustrated in FIG. 2, 300 g ofraw TDU (titer 94%), without treatment, containing 15 ppm of zinc, arecharged into and melted in the melter V1 at a temperature of 175° C. andat atmospheric pressure.

[0129] Operating under the conditions described in example 3, molten TDUis fed by means of P1 at a flow-rate of 307 g/hour.

[0130] After twenty minutes of feeding, there begins to be a differencein pressure between the inlet and outlet of the reactor R1, which afterthirty minutes becomes such as to no longer allow the test to becontinued. This pressure difference is caused by the progressive foulingof the exit line of the gases from the reactor R1, owing to theformation of by-products which are formed with consistence in thepresence of zinc.

[0131] The presence of traces of zinc in the TDU also creates theformation of considerable quantities of high-boiling by-products duringthe evaporation phase of the TDU, which cause significant fouling of thefilm evaporator.

EXAMPLE 7

[0132] With reference to the equipment illustrated in FIG. 3, 153.4 g(1.257 moles) of toluenediamine 80/20 (TDA 80/20, mixture of 2,4-2,6-isomers in a proportion of 80/20), 1600 g of dimethylcarbonate (DMC,weight ratio DMC/TDA equal to 10.4) and 8.3 g of zinc acetate dihydrate(0.037 moles, 5.4% by weight with respect to the TDA 80/20, molar ratiocatalyst/TDA 80/20 equal to 0.03), are charged into the steel reactor R1having a useful volume of 3 liters. The reactor is then brought to 2.5absolute atmospheres, heated so as to maintain an internal temperatureof 160° C. for 1.5 hours and stirred at about 300 rpm.

[0133] The condenser EC1 is brought to a temperature of 120° C., whereasthe condenser EC2 is cooled to a temperature of 5° C.

[0134] After 1.5 hours the connection between the reactor and condenserEC1 is intercepted, the internal temperature is brought to 175° C. for afurther 2 hours (finishing phase).

[0135] The maximum pressure registered during the test proved to beequal to 9 absolute atm.

[0136] At the end of the reaction, a quantity of distillate equal to325.5 g was collected, of which 80.5 g of methanol, 5.5 g of methylacetate and 239.5 g of dimethylcarbonate.

[0137] The reaction is left to cool to 120° C. and a quantity of waterequal to 1.5 g is added. The mixture is left under stirring at 120° C.for about 30 minutes and is then filtered on a filter F1, made ofsintered steel, having an average pore diameter of 2 microns (pressureabout 5 absolute atm.).

[0138] The reaction mixture thus filtered, consisting of 1360.5 g ofDMC, 282 g of toluene dimethylurethane (TDU containing the two isomers2,4 toluene dimethylurethane and 2,6 toluene dimethylurethane in theweight ratio of 80/20) and 18 g of by-products, is sent by means of P1to the reactor R2 where 12 mg of H₃PO₄ at 85% are added and where themixture is maintained under stirring for 2 hours at a temperature of130° C.

[0139] At the end of the treatment, the above liquid mixture is fed, bymeans of P2, with a flow-rate of 350 g/hour to the evaporator EV1maintained at a temperature of 175° C. and at atmospheric pressure. Thedimethylcarbonate is condensed by EC3 and collected in V2, whereas themolten urethane, collected at the bottom of V3, with a titer of 94% byweight and with a zinc content of 15 ppm and a content ofdimethylcarbonate of less than 0.5% by weight, is fed by means of P3,with a flow-rate of 297 g/hour, to the evaporator EV2 where 279 g/hourof mixture are vaporized.

[0140] The condenser EC4 is cooled with cooling liquid to T=40° C., EC5is cooled with cooling liquid to T=−20° C., EV2 is heated withdiathermic oil to T=306° C. and R3 is electrically heated to a walltemperature of 456° C.

[0141] EV2, R3, EC4, EC5, V4, V5, V6 are brought to a residual pressurevalue of 70 mmHg, measured at the outlet of EV2.

[0142] A liquid stream, equal to 18 g/hour, having the followingcomposition, is collected in V4 from the bottom of EV2:

[0143] 48.5% by weight of TDU

[0144] 14% by weight of toluene monourethane monoisocyanate (TMI)

[0145] 37.5% by weight of heavy by-products (allophanates, ureas).

[0146] The gas leaving R3 (279 g/hour) is partially condensed in EC4 andthe liquid obtained, equal to 206 g/hour, collected in V5, has thefollowing composition:

[0147] 89.7% by weight of toluene diisocyanate

[0148] 4% by weight of TMI

[0149] 6.2% by weight of ureas

[0150] 0.1% by weight of TDU.

[0151] The non-condensed gas in EC4, equal to 73 g/hour, containingmethanol, is condensed in EC5 and collected in V6.

[0152] In the final step, the TDI is recovered from the mixturecollected in V5 by distillation in the apparatus illustrated in FIG. 3and already described in example 5.

[0153] The condenser EC6 is brought to T=20° C., EV3 is heated to T=125°C. and all the distillation equipment consisting of EV3, C1, EC6, V7,V8, is maintained at a residual pressure value of 5 mmHg.

[0154] Operating under these conditions, the mixture contained in V5 isfed to EV3, by means of P4, with a flow-rate of 70 g/hour. The vaporleaving C1 (49 g/hour) is completely condensed in EC6 and the liquidobtained, collected in V8, consists of TDI with a purity of over 99.5%by weight.

[0155] A liquid stream, equal to 21 g/h, having the followingcomposition, is collected in V7 from the bottom of EV3:

[0156] 66.8% by weight of TDI

[0157] 12.2% by weight of TMI

[0158] 20.7% by weight of ureas

[0159] 0.3% by weight of TDU.

1. An integrated process for the preparation of aromatic isocyanatescomprising: the reaction between an aromatic amine and an organiccarbonate in the presence of a catalyst selected from organic andinorganic salts of a metal selected from Zn, Sn, Pb, Cu; the removal ofthe catalyst; the passivation of the residual quantities of metal in theurethane formed in step a); the removal of the solvent and its optionalrecycling to step a) of the reaction; the evaporation of the aromaticurethane with partial pyrolysis; the complete pyrolysis of the urethanein gas phase; the recovery of the isocyanate.
 2. The integrated processfor the preparation of aromatic isocyanates according to the previousclaim, wherein the aromatic amine fed to the initial reaction with theorganic carbonate corresponds to the formula: R—(NH₂)_(n) wherein n isan integer ranging from 1 to 2, R represents an aryl radical, as such orcontaining one or more substituents selected from alkyl radicals havingup to 4 carbon atoms, or alternatively atoms or radicals inert withrespect to the isocyanate function, selected from halogens, alkoxy,nitro, cyano, acyl, acyloxy, isocyanate groups.
 3. The integratedprocess for the preparation of aromatic isocyanates according to theprevious claim, wherein the aromatic amine is preferably selected from2,4-diaminotoluene, 2,6-diaminotoluene, aniline, toluidine,3,5-dichloroaniline, 4,4′-methylenedianiline, 2,4′-methylenedianiline,2,2′-methylenedianiline or mixtures of the above isomers.
 4. Theintegrated process for the preparation of aromatic isocyanates accordingto claim 1, wherein the organic carbonate is reacted with the aromaticamine in quantities equal to or higher than the stoichiometric value. 5.The integrated process for the preparation of aromatic isocyanatesaccording to claims 1 and 4, wherein the organic carbonate is selectedfrom alkyl esters of carbonic acid, with the ester group containing analkyl group having up to 6 carbon atoms.
 6. The integrated process forthe preparation of aromatic isocyanates according to the previous claim,wherein the alkyl group of the ester group preferably contains up to 4carbon atoms.
 7. The integrated process for the preparation of aromaticisocyanates according to claim 1, wherein the reaction between thearomatic amine and the organic carbonate is carried out in the presenceof a catalyst preferably selected from anhydrous or dihydrate zinccarboxylates, copper base carbonates, zinc base carbonates, mixed zincand copper carbonates, zinc carbamates.
 8. The integrated process forthe preparation of aromatic isocyanates according to claims 5 and 6,wherein the organic carbonate is preferably selected from dimethylcarbonate, diethyl carbonate, dipropyl carbonate, methyl butylcarbonate.
 9. The integrated process for the preparation of aromaticisocyanates according to claim 1, wherein the reaction between thearomatic amine and the organic carbonate is carried out by continuouslyremoving the alcohol which is formed, maintaining a quantity thereofranging from 10 to 40% molar with respect to the total co-product,inside the reaction mixture.
 10. The integrated process for thepreparation of aromatic isocyanates according to claim 1, wherein thereaction between the aromatic amine and the organic carbonate is carriedout in the presence of a solvent which is inert with respect to thereagents under the operating conditions.
 11. The integrated process forthe preparation of aromatic isocyanates according to the previous claim,wherein the solvent is selected from alkylated and non-alkylatedaromatic hydrocarbons, aromatic hydrocarbons containing functionalgroups inert to the reagents, alkanes as such or containing functionalgroups inert to the reagents.
 12. The integrated process for thepreparation of aromatic isocyanates according to the previous claim,wherein the solvent is preferably selected from benzene, toluene,xylene, anisole, benzonitrile, chlorobenzene, dichlorobenzene,cyclohexane, n-heptane, n-hexane, dichloromethane, diethylether,acetonitrile, dioxane.
 13. The integrated process for the preparation ofaromatic isocyanates according to claim 1, wherein the reaction betweenthe aromatic amine and the organic carbonate is carried out in thepresence of a quantity of catalyst ranging from 20 to 0.5% in moles permole of amine.
 14. The integrated process for the preparation ofaromatic isocyanates according to the previous claim, wherein theconcentration of catalyst preferably ranges from 10 to 1.0% in moles permole of amine.
 15. The integrated process for the preparation ofaromatic isocyanates according to claim 1, wherein the reaction betweenthe aromatic amine and the organic carbonate is carried out at atemperature ranging from 100° C. to 200° C.
 16. The integrated processfor the preparation of aromatic isocyanates according to the previousclaim, wherein the reaction temperature between the aromatic amine andthe organic carbonate preferably ranges from 140° C. to 180° C.
 17. Theintegrated process for the preparation of aromatic isocyanates accordingto claim 1, wherein the reaction between the aromatic amine and theorganic carbonate is carried out at a pressure ranging from 2 to 15absolute atm.
 18. The integrated process for the preparation of aromaticisocyanates according to the previous claim, wherein the pressure of thereaction between the aromatic amine and the organic carbonate preferablyvaries from 3 to 7 absolute atm.
 19. The integrated process for thepreparation of aromatic isocyanates according to claim 1, wherein thereaction between the aromatic amine and the organic carbonate isfollowed by the removal of the mixture consisting of the alcoholby-product and the excess organic carbonate, and by the subsequentseparation of this mixture with the organic carbonate recycled to thereaction itself.
 20. The integrated process for the preparation ofaromatic isocyanates according to claim 1, wherein the removal of thecatalyst used in the reaction between the aromatic amine and the organiccarbonate is favored by the addition of water.
 21. The integratedprocess for the preparation of aromatic isocyanates according to theprevious claim, wherein the water is added in a quantity ranging from0.5 to 10 moles of water per mole of catalyst initially charged.
 22. Theintegrated process for the preparation of aromatic isocyanates accordingto the previous claim, wherein the water is preferably added in aquantity ranging from 1 to 4 moles per mole of catalyst.
 23. Theintegrated process for the preparation of aromatic isocyanates accordingto claims 20, 21 or 22, wherein the addition of water is carried outunder the same conditions as the reaction between the aromatic amine andthe organic carbonate.
 24. A process for the synthesis of aromaticurethanes consisting in reacting an organic carbonate with an aromaticamine in the presence of a suitable catalyst characterized in that saidreaction is followed by the addition of water to favor the removal ofthe catalyst before the recovery of the urethane.
 25. The process forthe synthesis of aromatic urethanes according to the previous claim,wherein the water is added in a quantity ranging from 0.5 to 10 molesper mole of catalyst initially charged.
 26. The process for thesynthesis of aromatic urethanes according to the previous claim, whereinthe water is added in a quantity preferably ranging from 1 to 4 molesper mole of catalyst initially charged.
 27. The process for thesynthesis of aromatic urethanes according to claim 24, wherein the wateris added directly to the reaction mixture at temperatures ranging from100 to 200° C. and at a pressure ranging from 2 to 15 absolute atm. 28.The process for the synthesis of aromatic urethanes according to theprevious claim, wherein the water is added directly to the reactionmixture at temperatures preferably ranging from 110 to 160° C. and at apressure ranging from 3 to 7 absolute atm.
 29. The integrated processfor the preparation of aromatic isocyanates according to claim 1,wherein the passivation of the catalytic residues is effected bytreatment of the urethane formed with an acid selected from phosphoricand oxalic acid.
 30. The integrated process for the preparation ofaromatic isocyanates according to the previous claim, wherein, wheneffecting the treatment with phosphoric acid, the urethane containingcatalytic residues is dissolved in a low-boiling solvent in a weightratio with respect to the urethane ranging from 1:1 to 10:1.
 31. Theintegrated process for the preparation of aromatic isocyanates accordingto the previous claim, wherein the ratio between the solvent andurethane preferably ranges from 2:1 to 6:1.
 32. The integrated processfor the preparation of aromatic isocyanates according to claim 29,wherein, when effecting the treatment with phosphoric acid, saidtreatment is carried out at a temperature ranging from 100° C. to 150°C. for a period of time ranging from 0.5 to 4 hours.
 33. The integratedprocess for the preparation of aromatic isocyanates according to theprevious claim, wherein the treatment is carried out at a temperaturepreferably ranging from 120° C. to 140° C.
 34. The integrated processfor the preparation of aromatic isocyanates according to claim 32,wherein the treatment is carried out for a period of time preferablyranging from 1 to 2 hours.
 35. The integrated process for thepreparation of aromatic isocyanates according to claim 29, wherein, wheneffecting the treatment with phosphoric acid, said treatment is carriedout under the same pressure conditions as the reaction between aromaticamine and organic carbonate.
 36. The integrated process for thepreparation of aromatic isocyanates according to one or more of theclaims from 30 to 35, wherein the low-boiling solvent is preferablyselected from dimethylcarbonate, diethylcarbonate, tetrahydrofuran,dioxane, acetonitrile, methanol, ethanol.
 37. The integrated process forthe preparation of aromatic isocyanates according to the previous claim,wherein the treatment is preferably carried out in the same solvent inwhich the synthesis of urethane is effected.
 38. A process for thepreparation of aromatic isocyanates by the pyrolysis of thecorresponding urethanes consisting in subjecting the urethanes, in themolten state or in solution, to thermal treatment under such temperatureand pressure conditions as to allow the vaporization and partialconversion into isocyanates, and subsequently subjecting the vapors topyrolysis.
 39. The process for the preparation of aromatic isocyanatesby the pyrolysis of the corresponding urethanes according to theprevious claim, wherein the urethane is previously treated with an acidfor the passivation of the catalytic residues.
 40. The process for thepreparation of aromatic isocyanates according to the previous claim,wherein the urethane is treated with phosphoric acid after dissolutionin a low-boiling solvent.
 41. The process for the preparation ofaromatic isocyanates according to the previous claim, wherein theurethane is dissolved in a solvent with a weight ratio between thelatter and the urethane ranging from 1:1 to 10:1.
 42. The process forthe preparation of aromatic isocyanates according to the previous claim,wherein the weight ratio between the solvent and the urethane preferablyranges from 1:1 to 3:1.
 43. The process for the preparation of aromaticisocyanates according to claim 40, wherein the treatment with phosphoricacid is carried out at a temperature ranging from 100° C. to 150° C. fora period of time ranging from 0.5 to 4 hours.
 44. The process for thepreparation of aromatic isocyanates according to the previous claim,wherein the treatment is carried out at a temperature preferably rangingfrom 120° C. to 140° C.
 45. The process for the preparation of aromaticisocyanates according to claim 43, wherein the treatment is carried outfor a period of time preferably ranging from 1 to 2 hours.
 46. Theprocess for the preparation of aromatic isocyanates according to one ormore of claims 40 to 45, wherein the solvent is selected fromdimethylcarbonate, diethylcarbonate, tetrahydrofuran, dioxane,acetonitrile, methanol, ethanol.
 47. The process for the preparation ofaromatic isocyanates according to the previous claim, wherein thesolvent is preferably the same as that of the urethane synthesis. 48.The integrated process for the preparation of aromatic isocyanatesaccording to claim 1, wherein the organic solvent is removed bydistillation.
 49. The integrated process for the preparation of aromaticisocyanates according to the previous claim, wherein the removal of thesolvent is carried out at a temperature ranging from 100° C. to 200° C.50. The integrated process for the preparation of aromatic isocyanatesaccording to the previous claim, wherein the temperature preferablyranges from 150° C. to 180° C.
 51. The integrated process for thepreparation of aromatic isocyanates according to claim 48, wherein theremoval of the solvent is carried out at a pressure ranging from 3 to0.2 absolute atm.
 52. The integrated process for the preparation ofaromatic isocyanates according to the previous claim, wherein thepressure is preferably selected within the range of 2 to 0.5 absoluteatm.
 53. The integrated process for the preparation of aromaticisocyanates according to claim 48, wherein the removal of the solvent iscarried out in an apparatus consisting of one or more evaporators with ashort contact time.
 54. The integrated process for the preparation ofaromatic isocyanates according to claim 1, wherein the aromatic urethaneobtained down-stream of the passivation of the catalytic residues andremoval of the solvent is subjected, in the molten state or in solution,to an evaporation treatment with partial pyrolysis and to subsequentpyrolysis in gas phase.
 55. The integrated process for the preparationof aromatic isocyanates according to claim 54, or the process for thepreparation of aromatic isocyanates according to claim 38, wherein theurethanes, in the molten state or in solution in an inert high-boilingsolvent, are fed to a thermal treatment at a temperature ranging from230° C. to 380° C., at a pressure ranging from 1 to 300 mmHg and at aLSHV space velocity ranging from 0.2 to 4 hours⁻¹.
 56. The processaccording to any of the indications of claim 55, wherein the urethanepreferably consists of 2,4-toluene dimethylurethane, 2,6-toluenedimethylurethane, mixtures of 2,4-toluene dimethylurethane and2,6-toluene dimethylurethane, N-phenylmethylurethane,N-phenylethylurethane, 4-methylphenylethylurethane, 3,5-dichlorophenylethylurethane, 4,4′-methylene bis(phenylmethylurethane),2,4′-methylene bis(phenylme-thylurethane), or mixture of isomers. 57.The process according to any of the indications of claim 55, wherein thetreatment is carried out at a temperature preferably ranging from 270°C. to 320° C.
 58. The process according to any of the indications ofclaim 55, wherein the treatment is carried out at a pressure preferablyranging from 20 to 150 mmHg.
 59. The process according to any of theindications of claim 55, wherein the treatment is carried out at a LSHVspace velocity preferably ranging from 0.5 to 2 hours⁻¹.
 60. The processaccording to any of the indications of claim 55, wherein the treatmentis continued by feeding the mixture of vapors produced by a pyrolysis ata temperature ranging from 300° C. to 600° C., in equipressure with theprevious treatment.
 61. The process according to the previous claim,wherein the pyrolysis treatment is carried out at a temperaturepreferably ranging from 350° C. to 550° C.
 62. The process according toclaim 60, wherein the feeding of the vapors takes place at a GHSV spacevelocity ranging from 20 to 500 hours⁻⁴.
 63. The process according tothe previous claim, wherein the feeding of the vapors takes place at aspace velocity preferably ranging from 40 to 200 hours⁻¹.
 64. Theprocess according to any of the claims from 60 to 63, wherein thetreatment is continued by sending the mixture of vapors leaving thepyrolysis to fractionated condensation.
 65. The process according to theprevious claim, wherein the fractionated condensation is carried out bymeans of at least two steps.
 66. The process according to the previousclaim, wherein the first condensation step is carried out at atemperature ranging from 10 to 150° C.
 67. The process according toclaim 65, wherein the second condensation step is carried out at atemperature ranging from −80 to +50° C.
 68. The process according toclaim 66, wherein the first condensation is preferably carried out at atemperature ranging from 20° C. to 100° C.
 69. The process according toclaim 67, wherein the second condensation is preferably carried out at atemperature ranging from −30° C. to +10° C.
 70. The process according toany of the indications of claim 55, wherein, when the urethane is fed toevaporation in a solution of an inert high-boiling solvent, the latteris used in a weight ratio with respect to the urethane ranging from 3:1to 0.01:1.
 71. The process according to the previous claim, wherein theratio between the high-boiling solvent and the urethane preferablyranges from 0.3:1 to 0.05:1.
 72. The process according to any of claims70 and 71, wherein the solvent is selected from polyphenyls, triphenyl,tetraphenyl, dodecylbenzene, dibenzyltoluene, polyphenylether,methylnaphthalene, benzylnaphthalene, dichloronaphthalene,dibutylphthalate, dioctylphthalate, diphenylsulfone, phenyltolylsulfone,naphthylphenylsulfone.
 73. The process according to any of theindications of claim 55, wherein the thermal treatment is carried out ina reactor consisting of a fine film evaporator.
 74. The processaccording to the previous claim, wherein part of the product fed isvaporized and the remaining fraction is discharged at the bottom. 75.The process according to the previous claim, wherein the ration betweenthe vaporized product and the product collected at the bottom rangesfrom 70:30 to 99:1.
 76. The process according to the previous claim,wherein the ration between the vaporized product and the productcollected at the bottom preferably ranges from 80:20 to 95:5.
 77. Theprocess according to claim 74, wherein the product collected at thebottom, after separation of the pitches, is fed to pyrolysis treatment.78. The process according to claim 60, wherein the treatment is carriedout in a tubular reactor.
 79. The integrated process for the preparationof aromatic isocyanates according to claim 54, or the process for thepreparation of aromatic isocyanates according to claim 38, wherein theprocess continues by recovering the isocyanate in a concentrationranging from 80 to 99% by weight.
 80. The process according to theprevious claim, wherein the isocyanate is recovered by distillation. 81.The process according to the previous claim, wherein the distillation iscarried out in an apparatus comprising an evaporator and a column. 82.The process according to claim 80, wherein the distillation is carriedout at a temperature ranging from 60 to 200° C. and at a pressureranging from 1 to 200 mmHg.
 83. The process according to the previousclaim, wherein the distillation is carried out at a temperaturepreferably ranging from 90 to 150° C.
 84. The process according to claim82, wherein the distillation is carried out at a pressure preferablyranging from 3 to 40 mmHg.
 85. The process according to any of theclaims from 79 to 84, wherein the liquid remaining at the bottom of thecolumn is recycled to the treatment according to claim
 55. 86. Theprocess according to the previous claim, wherein the liquid remaining atthe bottom of the column is previously treated with the alcoholcorresponding to that formed in the isocyanate production reaction. 87.The process according to the previous claim, wherein the treatment iscarried out at a temperature ranging from 25 to 90° C.
 88. The processaccording to claim 86, wherein the treatment is carried out for a periodof time ranging from 0.5 to 3 hours.
 89. The process according to claim86, wherein the treatment is carried out with a quantity of alcoholwhich is such that the weight ratio between this and the liquid to betreated ranges from 2:1 to 10:1.